or mains socket with an earth connection/pin

‘prise au secteur’ is just a mains/electrical socket found in the home (Routledge).

The natural way to say the phrase would be “only plug the unit into a socket with an earth pin”.

The phrase probably seems strange to the British context as all domestic electric sockets are earthed (save for isolated electric shaver sockets) in the UK (US I don’t know about). However, in France some sockets have an earth connection, and some do not. For those that do it is in the form of an earth pin. The failure of this system is that equipment that needs to be earthed can still be plugged into an unearthed socket, hence the instruction.


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Note added at 2 hrs (2008-04-16 21:02:11 GMT)
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In fact if the translation is being done to sell the unit in question on the British market the phrase is essentially redundant, or at best gets reduced to ‘Plug the unit in’ (to an electric socket goes without saying). And to talk of a socket with an earth pin would just serve to confuse. A note to the client might be in order.